1. Field of the Invention
The present invention relates to a developing device, an image forming apparatus, and a process cartridge, and in particular to a developing mechanism, which employs two-component developer having toner and carrier.
2. Description of the Background Art
In an image forming apparatus, such as a copier, a printer, a facsimile machine, a duplicator, etc., a latent image is formed on a photoconductive member and is visualized by a developing device. The visualized image is then transferred onto a sheet or the like as a printing output.
One- or two-component developer including only magnetic or non-magnetic toner or a mixture of such toner and carrier can be used.
The toner in the two-component developer is charged triboelectrically by frictional electrification when stirred and mixed, to be attracted to a latent image formed on a photoconductive member.
The developing device includes a developing sleeve that carries and supplies developer to a latent image on a photoconductive member. The developing sleeve causes the developer particles to form upright ears on its circumferential surface. The developing device also includes a stirring member, such as a screw auger, etc., that stirs, mixes, and supplies the developer to the developing sleeve. The developer on the developing sleeve is flattened by a doctor blade or the like to form a layer having a prescribed thickness and amount, and is supplied to a latent image on the photoconductive member.
As described in Japanese Patent Application Laid Open No. H11-258909 (JP-H11-258909-A), it is known that developing quality generally varies depending on an amount of developer used in developing and/or remaining amount of the developer downstream of a developing region.
Accordingly, it has been attempted in the past that a developing history is erased by peeling off the remaining developer downstream of the developing region while supplying fresh developer as described in JP-H11-258909-A. Specifically, a developing roller having a magnetic recording layer on its circumferential surface is provided to convey developer to a region of the circumferential surface facing an image bearer.
Further provided is a peeling-off roller downstream from the developing region. The peel-off roller includes a non-magnetic sleeve and plural rotatable magnets installed therein. The peeling-off roller closely approaches the circumferential surface of the developing roller to peel off multiple layers of the developer formed thereon.
The above-mentioned peeling off operation is achieved by the mechanism described below.
When the same images are successively developed in previous developing cycles, developer carried on the developing roller remains unpeeled. Thus, when the next developing cycle is executed while carrying fresh developer, a developing pattern formed in the previous developing cycle appears as a history.
As a result, toner density, charge amount, and particle diameter or the like (corresponding to the developing pattern) become different in a region with the developing history from those in another region without the same unless the history is erased. Specifically, the developing pattern causes a ghost on the next image (especially, a halftone image) during the next developing cycle, that is otherwise generally absent in an image formation area, thereby likely exacerbating image reproducibility and quality.
Then, to erase the developing history with the structure as described in the JP-H11-258909-A, the developer must be peeled off from the developer carrier.
However, since the above-mentioned peel-off roller is constructed of the non-magnetic sleeve and the plural rotatable magnets installed in the vicinity of the non-magnetic sleeve as described above, magnetic interference increasingly likely occurs between the developing roller side magnet and the peel-off roller side magnet as these magnets rotate.
Further, when vibration occurs between prescribed members due to the magnetic interference or the developer remains on the developing roller, an abnormal image having one or more stripes are likely created.